The present invention relates to radar apparatus and provides a means and method of determining whether a signal received by a main channel, particularly but not exclusively a sum channel, of an antenna of the apparatus has been received from a direction within a high gain main lobe of the antenna or whether it has been received from a direction lying outside the main lobe.
Many radar systems employ highly directional antennas, typically slotted planar waveguide array or phased array antennas, comprising a large number of antenna elements the outputs of which are summed to provide a sum channel output, which output has a characteristic gain pattern comprising a, normally central, main lobe and a number of sidelobes, the sidelobes typically decreasing in amplitude the greater the angle from the main lobe.
In addition to a sum channel, a radar antenna may also have one or more difference channel outputs. These are obtained by separately summing the outputs of the antenna elements in different regions of the antenna, typically four quadrants, and taking the difference between the signal received from two regions. This provides an interferometric technique which can provide an angular discriminant for tracking purposes.
A signal received from a small target in a direction corresponding to the main lobe of a sum channel of an antenna will provide a signal of similar intensity to a larger target in a direction lying outside the main lobe. Therefore it is not possible to use returned signal strength to determine whether a target lies in a direction of the main lobe or not.
The simplest way of determining the direction in which the target lies relative to the antenna is to scan the antenna noting the position of the main lobe when the maximum signal is received, the direction of the main lobe at this instant corresponding to the direction of the target. This however is often impracticable to implement or is ineffective, especially if the apparatus is operating in a mode such that its measurements of range and/or doppler frequency are ambiguous.
Guard antennas are widely used in primary and secondary radar applications to provide a means of distinguishing between main beam and sidelobe radar echoes. Guard antennas are normally relatively small in comparison with the main antenna and have a wide angle beam of substantially uniform gain relative to the main antenna. However guard antennas may sometimes be integral with the main antenna The signals received from the main antenna and guard antenna are compared, FIG. 1 of the attached drawings illustrating a comparison of the relative gain of the two antennas. If the signal received from the main antenna M(z) is greater than the signal received from the guard antenna G(z) then the target must be located in a direction corresponding to the main lobe of the main antenna, and vice versa. This can be expressed as:
main antenna gain M(z) greater than guard antenna gain G(z)=within the main beam main antenna gain M(z) less than guard antenna gain G(z)=outside the main beam
where z defines the angle at which the comparison is made.
In practice, conventional antenna configurations typically use a large, high gain main antenna and a small, broad beam wide-angle guard antenna. This type of implementation often provides inadequate G(z)/M(z) gain margin in the region A of the close-in sidelobes of the main antenna, as illustrated in FIG. 2, where the gain of the guard antenna G(z) is less than the gain of the main antenna M(z).
If to overcome the above problem the gain G(z) of the guard antenna is modified such that it has a gain pattern as illustrated in FIG. 3, then while the gain pattern is sufficient such as to ensure the gain of the guard antenna is greater than the gain of the main antenna in the close-in sidelobe regions A, the gain of the guard antenna falls below that of the main antenna in the far-out regions B such that it is not possible to determine from a comparison of the main antenna and guard antenna signals whether a signal has been received in a direction corresponding to the main lobe of the main antenna C or whether that signal has been received from the far-out regions B.
One way in which it has been proposed to overcome this problem is to employ two guard antennas, the first being a small wide-angle antenna having a uniform G1(z), as illustrated in FIG. 4, and the second, a larger directional antenna, having a gain G2(z). By summing the power output of the two guard antennas the desired guard antenna gain is achieved, as shown by the broken line in FIG. 4. If the signal received from the main antenna is greater than that of the combined channel of the guard antennas then the signal can unambiguously be classified as having been received in a direction corresponding to the main beam of the main antenna. The problem with this arrangement however is that it is necessary to employ both a small wide-angle guard antenna and also a larger directional guard antenna, substantially increasing the cost of such an arrangement, and often more importantly the bulk of the radar apparatus.
According to the present invention there is provided radar apparatus comprising an antenna having a plurality of antenna elements from which a plurality of output signals can be derived having different gain characteristics;
means for deriving from a first set of plurality of antenna elements a first output channel having a relatively high gain main lobe;
means for deriving from a second set of the plurality of antenna elements a second output channel having a different gain pattern, at least some of the plurality of antenna elements being common to the first and second sets;
means for deriving one or more further output channels each having a different gain pattern to the first and second output channels; and
a discriminator for generating a sidelobe suppression signal from the signals received on the second and one or more further output channels, the discriminator determining that the direction from which a signal is received by the antenna is in the main lobe of the first channel by comparison of the signal received on the first channel with the value of the sidelobe suppression.
The present invention arises from the realisation that channels other than the main channel of an antenna, the difference channel for example, have a high gain in the region corresponding to the relatively high gain sidelobes of the sum channel but have a null on the antenna boresight, the peak of the sum channel response. The difference channel for example can thus be used to provide at least in part the desired sidelobe suppression signal for blanking the sidelobe.
Preferably the antenna comprises a plurality of elements interconnected as a number of arbitrary groups, the output signals from the groups being combined in different manners such as to produce the first and second output channels.
Detailed examination of typical antenna patterns have now shown that the difference channel gain patterns possess the desirable gain margins for generating sidelobe suppression signals on or near the principal or cardinal planes of the antenna, but normally have inadequate gain in regions well off the boresight and in the intercardinal planes. By contrast small low gain guard antennas provide good coverage of sum channel sidelobes in the far-out and intercardinal regions, but generally show inadequate gain margin in the close-in sidelobe regions, and it is preferable that the apparatus further comprises such a guard type antenna having a wide beam relatively uniform gain pattern, the output of which is combined with the output of the second channel to provide the sidelobe suppression signal. This may be done by analogue or digital processing means.
It may be advantageous if the discriminator applies scaling factors to the second output channel and a further output channel to set their relative gains in the most suitable position relative to the first channel.
Depending on the application to which the antenna is put, it may be desirable for the second channel to be a ring difference channel, as this channel particularly possesses the desired gain margins.
Advantageously the antenna comprises a slotted planar waveguide array or phased array antenna consisting of a plurality of elements interconnected such as to define two groups, the output signals of the two groups being summed to obtain a sum channel, and differenced to obtain a difference channel. Preferably the antenna configuration comprises an antenna divided into four quadrants, each quadrant being defined by having a plurality of antenna elements the outputs of which are summed, the apparatus comprising means for generating from the difference in output of two pairs of horizontally separated quadrants an azimuth difference channel, and means for generating from the difference in output of two pairs of vertically separated quadrants an elevation difference channel. Advantageously each channel has a respective receiver associated with it. Alternatively the elevation difference channel and azimuth difference channel may be alternately switched to a common receiver.
According to a second aspect of the present invention there is provided a method of determining whether a signal received by radar apparatus is received from a direction within a high gain main lobe of a first channel of an antenna of the radar apparatus, the method comprising:
deriving a first output channel from a first set of a plurality of elements of the antenna with a relatively high gain pattern having a main lobe;
deriving from a second set of the plurality of elements of the antenna, a second output channel having a different gain pattern, some elements being common to the first and second sets;
deriving one or more further output channels each having a different gain pattern to the first and second output channels;
generating a sidelobe suppression signal, the value of which is determined at least in part from the signal received on the second and one or more further output channels; and
determining that the direction from which a signal is received by the antenna is in the main lobe of the first channel by comparison of the value of a signal received on the first channel with the value of the sidelobe suppression signal.
One embodiment of the invention will now be described by way of example only with reference the accompanying drawings, of which: